This invention relates generally to apparatus and methods for removing lining from tubing. In a particular aspect, this invention relates to removing polyurethane lining from casing used in wells.
In constructing a well from which liquid or gas is to be produced, various types of tubing strings can be put in the drilled borehole. One type is typically called "casing." Traditionally this has been a metal tubing having a relatively large inner diameter that allows other tubing strings to be lowered through it. One way to use casing is to lower it into the borehole and then pump cement such that the cement is placed in the annulus between the casing and the wall of the borehole. These operations are performed using well-known techniques.
Another type of tubing string that has been used is a smaller diameter string that is run into the well inside previously installed casing. Such a narrower string might be used to produce oil or gas from the well to the surface, for example. Another example is that such a string might be used to inject substances into the well, such as in a technique referred to as "secondary recovery" in which the injected substance pushes hydrocarbons out of the well (or out another well or other wells).
More recently, a different type of casing has been used in some applications. This type of casing includes the traditional metal tubing, but one lined with plastic. The plastic lining is typically made of a thermoplastic polymer, a non-limiting example of which is polyurethane. With this type of casing, some substances can be produced from or injected into a well without the use of the traditional inner production or injection tubing string referred to in the immediately preceding paragraph. The inner diameter of the lined casing is larger than the inner diameter of the traditional production or injection tubing; therefore, more production or injection per unit of time can be obtained through the lined casing alone than through the narrower traditional production or injection string. That is, higher volumetric flow rates can be obtained through the lined casing. This type of casing has been used, for example, in producing gaseous carbon dioxide from a first well and in injecting it into a second well in a secondary recovery process for driving liquid or gaseous hydrocarbons out of the second well or out of the formation intersected by the second well.
The lined casing application referred to above, in which no separate inner tubing string is used, has advantages over the traditional casing plus production/injection string technique. In addition to the larger flow advantage mentioned above, the lined casing can be used less expensively. Furthermore, the lining is more resistant to corrosion than the metal casing. Such lining can be used to cover damaged casing walls.
Although there are at least the aforementioned advantages, the plastic lining can be damaged during installation and sometimes the metal casing may corrode or deteriorate sufficiently that it needs to be repaired even though it may be covered by the lining. When this damage or deterioration occurs, the lining needs to be pulled out of the outer metal tubing and a new lining installed (and possibly repairs made to damaged metal tubing). Although the outer metal tubing is typically cemented into the well borehole, the lining is retained in the metal tubing by its own outwardly directed force and friction. That is, the lining is not glued or otherwise separately adhered to the metal tubing. Rather, the lining is inserted in known manner into the metal tubing in a radially inwardly compressed state; once installed, the resilient lining (having an uncompressed outer diameter larger than the inner diameter of the metal tubing) expands against the inner surface of the metal tubing so that the lining is held by the radially outward force exerted by the lining and friction between the outer surface of the lining and the inner surface of the tubing. At the mouth of the well, a plastic flange is fused to the upper end of the lining to also provide support. In view of the foregoing reasons why lining sometimes needs to be removed, and since the outer metal tubing is cemented in the borehole, there are the need for a system and method for removing lining from the tubing and the need for components for such system and method.